While common, I don't think this is a standard, so it will vary by service provider or email server. According to spec, "+" can be a valid part of an email address:

    Without quotes, local-parts may consist of any 
    combination of alphabetic characters, digits, or any of 
    the special characters

        ! # $ % & ' * + - / = ?  ^ _ ` . { | } ~

https://tools.ietf.org/html/rfc3696#page-5

I’ve had many experiences where one day a site will stop handling + correctly and I can no longer sign into my account. More common is the registration form allows + but the sign in does not, or I can subscribe to an email newsletter but not unsubscribe.

As far as the service is concerned, that's simply an entirely different email address. You might as well do [email protected]. Since you control domain.tld, you can of course receive any email sent to it.

But the point the GP was making is that email addresses are treated by services as if the will always be tied to the same person, but that's simply not true.

This only works with Gmail, and copycats